2.2.0_arenadata2

fix updating from 1.x to 2.y when y > 0 (#12)

Functions in the SQL script for each diskquota 2.x version refer to the exact
name of shared library, e.g. for diskquota-2.1.so. As a result, upgrade from
2.0 to 2.2 is impossible without presence of 2.1 shared library as far as each
extension function validated by loading shared library and checking symbol
name. This patch implement symlinks creation logic to coup this problem.
Symlinks for each previous release to the latest shared library will be
created. But we need to control changing of sql function during next syncs.